In pulley-belt systems, it is known practice to use a rolling-element bearing to guide the movement of a pulley that is driven in rotation by a belt. Such a pulley comprises an inner web for connecting to an outer rolling-bearing race and an outer web which cooperates with the belt. Radial ribs connect the inner web to the outer web in order to stiffen the assembly. In conventional pulleys, these ribs are distributed at a regular angle around a rotation axis of the pulley. However, this can result in significant vibration when the pulley goes into resonance that is to say at certain particular speeds. Therefore, this type of pulley is not suitable for an automotive application since the emission of noise in the passenger compartment is a source of discomfort for the passengers.
U.S. Pat. No. 5,630,769 discloses what is referred to as a low-noise pulley, in which the ribs are distributed irregularly around the rotation axis of the pulley.
Moreover, US-A-2011/030 09 79 likewise discloses what is referred to as a low-noise pulley, comprising two sets of ribs which are disposed on either side of a midplane of the pulley. This midplane is orthogonal to a rotation axis of the pulley. The first and second sets of ribs have the same number of ribs. In addition, the ribs of the first and second sets are distributed at a regular angle around the rotation axis of the pulley. Finally, the ribs of the first set are angularly offset with respect to the ribs of the second set.
The angular distribution of the ribs of the pulley in U.S. Pat. No. 5,630,769 or in US-A-2011/030 09 79 does not make it possible to reduce the level of noise generated during operation below an acceptable threshold.
It is these drawbacks that the invention is intended more particularly to remedy by proposing a pulley for further reducing the noise associated with operation.
To this end, the invention relates to a pulley intended to be mounted around a rolling bearing, this pulley comprising an inner web for connecting to an outer race of the rolling bearing, an outer web for connecting to a flexible force-transmitting element, and radial connecting ribs between the outer web and the inner web. According to the invention, the ribs are distributed in several groups, each group comprising at least three successive ribs, which form several pairs of adjacent ribs, the midplanes of which are separated two by two at a regular angle within the group, and the angle between the midplanes of two adjacent ribs in one and the same group varies, between two adjacent groups, by at least 5% with respect to the smaller of the two angles.
By virtue of the invention, the particular distribution of the ribs causes the pulley to vibrate less at certain particular speeds, such as the resonance speed. The noise generated by the pulley is thus acceptable, and so the pulley can be incorporated into a motor vehicle. Specifically, the noise then generated inside the passenger compartment does not bother the passengers.